I. Field of the Invention
This invention relates generally to the design of microstrip antenna arrays for transmitting and receiving millimeter wave signals, and more particularly to a microstrip antenna system incorporating physically interleaved, but electrically independent broadside antenna arrays which are relatively stable in performance irrespective of variations in temperature and frequency.
II. Discussion of the Prior Art
As is pointed in applicant's co-pending application Ser. No. 172,461, filed Mar. 24, 1988, entitled "INTERLEAVED PRINTED CIRCUIT ARRAY ANTENNAS", various missile tracking and electronic countermeasure systems, as well as target-seeking weapons, employ radar and radiometric devices in which millimeter wave signals are sent and received via microstrip antenna arrays. Specifically, that application describes how two independent microstrip antenna arrays may be interleaved to occupy substantially the same physical space. In applicant's earlier arrangement, each of the two interleaved antenna arrays is edge fed from a single edge only. It is found that with shifts in temperature and/or in the transmitter frequency, the main beams of the two antennas tend to move in opposite directions of each other so as to no longer exhibit true broadside antenna characteristics. This is due to the fact that the transmission lines carrying the radiator elements act as travelling wave devices. A problem thus presents itself on just how to design a microstrip broadside antenna structure for millimeter wave applications in which a sending and receiving array can be interleaved but which will still exhibit a stable performance with variations in temperature and frequency.
It is accordingly a principal object of the present invention to solve the foregoing problem, i.e., to provide an antenna structure which is stable over frequency and temperature. I have discovered that by feeding a symmetric array of transmission line radiators in its center, the variation in main beam position over frequency and temperature of each half of the array can be made to cancel the variation induced in the other half of the array. Thus, such a center-fed array exhibits a stable main beam position over frequency and temperature. This suggests that a center feed is a desirable configuration, but because of space limitations, it is extremely difficult to physically interleave two center-fed arrays because the feed for each array interferes with the other. To overcome this difficulty, I have utilized a second, edge-fed array which is similar in many respects to a center-fed array but which the segments thereof are flipped so that the feedlines extend along the opposed outside edges rather than down the middle. By properly laying out the printed circuit masks used to create the microstrip antenna transmission lines and radiators, the spacing between transmission lines and radiators can by such that the two antenna arrays can be interleaved without physical interference (overlap) of the conductive patterns.